This invention relates to a process of forming a tin oxide coating on a hot glass substrate by chemical reaction and/or decomposition of tin halide supplied in the vapour phase.
There is a demand for glass products, in particular flat glass, having a tin oxide coating of high optical quality which modifies the radiation transmitting characteristics of the product but causes little or no diffusion of transmitted light. Any significant light diffusion within a transparent product is apparent as haze.
In the prior art it is recognised that the appearance of haze in coatings formed on glass by exposure to tin halide can be caused by alkali metal salt reaction products of the glass with the tin halide. In order to inhibit such reactions U.S. Pat. No. 2,617,741 proposed to precede the formation of the tin oxide coating by the formation of a protective layer which is free of Na.sub.2 O or which has a content of available Na.sub.2 O lower than that of the surface of the heated glass. The protective layer is formed, according to that United States Patent, by spraying the heated glass with a saturated or relatively concentrated aqueous solution of a suitable soluble metal salt, those mentioned including salts of copper, aluminium, lead, zinc, iron, nickel, cobalt, thalinum, silver and titanium. The United States specification states that the chlorides, bromides, iodides, nitrates and nitriles of those metals can be used but that clearest films are obtained when using the acetates.
It is a fact that haze can result from reactions between the tin halide and the glass substrate and that such undesirable reactions can be prevented or reduced by application of an intervening coating of suitably selected composition. But haze can of course occur which has other causes such as defects in the structure of the tin oxide coating or of the subjacent protective coating itself, if applied. Obviously it will be a normal objective to control coating conditions to try to achieve coatings which are as uniform in thickness and composition as possible.
Notwithstanding the application of a protective coating beneath the tin oxide coating and careful control of coating procedures the problem of haze formation persists. The problem varies in severity depending on numerous factors. These include the environmental conditions under which coating takes place, the coating thickness and the rate of its formation measured in terms of the growth in the coating thickness per unit time.
The aforesaid United States patent is concerned with the formation of electro-conductive coatings on glass sheets. Electro-conductive coatings are referred to which have a thickness of about 3000 to 4000 A. Research has shown that the difficulty of avoiding haze tends to increase as the coating thickness and the coating rate increase. The coating rate is of course an important factor in an industrial mass production plant. This is particularly so if it is required to coat a glass ribbon during its transit from a flat glass forming station.